The aim of this prospective study was to determine the prevalence of pes anserine bursitis (PAB) in patients with osteoarthritis. A total of 85 patients with primary knee osteoarthritis diagnosed according to the American College of Rheumatology (ACR) criteria were included in the study. The patients were divided into four groups using the Kellgren-Lawrence staging system. The knee X-rays evaluated according to this system indicated that 15.6 % of patients were grade 1, 34.1 % grade 2, 37.1 % grade 3, and 13.5 % grade 4. Ultrasonography (USG) was performed on both knees by a radiologist experienced in musculoskeletal system ultrasonography and unaware of the patients' physical examination or direct X-ray findings. The presence of PAB, longest diameter of bursitis, and area of bursitis were recorded. The average age of the 11 male and 74 female patients included in the study was 58.9 +/- 9.0 years. A total of 170 knees of 85 patients were examined. The incidence of bursitis was significantly higher in females (p = 0.026). The incidence of bursitis on ultrasound was 20 % (34/170). There was a statistical difference between the grades for bursitis incidence (p = 0.004). There was a significant positive correlation between both the longest length (p = 0.015, r = 0.187) and area (p = 0.003, r = 0.231) of PAB with osteoarthritis grade. The mean age of bursitis patients was higher than that of those without the condition (p = 0.038). In addition, the osteoarthritis (OA) grade and bursitis prevalence increased as the patients' age increased, and these increases were statistically significant (p < 0.001). PAB is easily evaluated with ultrasonography. Pes anserine bursitis was observed in one out of every five symptomatic OA patients and was more common in female patients and with advanced age. A positive correlation was found between OA grade and PAB size and area.